Natural, Proteolytic Release of a Soluble Form of Human IL-15 Receptor α-Chain That Behaves as a Specific, High Affinity IL-15 Antagonist

Immunological characterization of IgG subclass deficiency reveals decreased Tregs and increased circulating costimulatory and regulatory immune checkpoints

Background

Immunoglobulin G subclass deficiencies (IgGsd) comprise a wide clinical spectrum from no symptoms to repeated respiratory infections and risk for the development of lung damage. Our aims were to investigate whether the immunological phenotype of IgGsd patients on and off immunoglobulin replacement therapy (IgRT) was reflected in the clinical features of IgGsd.

Method

Thirty patients with IgGsd were included in this prospective study of 18 months of IgRT, followed by 7-18 months of IgRT discontinuation. Blood samples were collected when patients were on and off IgRT and compared with samples from 34 cross-sectional healthy controls. An in-depth lymphocyte phenotyping was performed by flow cytometry and plasma levels of immune checkpoints were assessed.

Results

IgG3 subclass deficiency was most common. Patients with IgGsd had decreased levels of activated T cells and B cells and plasma levels of negative immune checkpoint molecules correlated negatively with T cell and B cell activation. The decreased T cell activation level was unaffected by IgRT, while the B cell activation was partly restored. Of note, decreased levels of activated regulatory T cells (Tregs) were found in IgGsd patients and was partly restored during IgRT. The profile of comorbidities did not associate with Treg levels.

Discussion

IgGsd is associated with decreased B cell and T cell activation including Tregs, and increased plasma levels of negative immune checkpoint molecules. The consequence of reduced activated Tregs in IgGsd remains unclear. Decreased immune cell activation was partly restored during IgRT, demonstrating that IgRT may contribute to improved immune function in patients with IgGsd.

Profibrogenic role of IL-15 through IL-15 receptor alpha-mediated trans-presentation in the carbon tetrachloride-induced liver fibrosis model

Background

Inflammatory cytokines play key pathogenic roles in liver fibrosis. IL-15 is a proinflammatory cytokine produced by myeloid cells. IL-15 promotes pathogenesis of several chronic inflammatory diseases. However, increased liver fibrosis has been reported in mice lacking IL-15 receptor alpha chain (IL-15Rα), suggesting an anti-fibrogenic role for IL-15. As myeloid cells are key players in liver fibrosis and IL-15 signaling can occur independently of IL-15Rα, we investigated the requirement of IL-15 and IL-15Rα in liver fibrosis.

Methods

We induced liver fibrosis in Il15-/- , Il15ra-/- and wildtype C57BL/6 mice by the administration of carbon tetrachloride (CCl4). Liver fibrosis was evaluated by Sirius red and Mason's trichrome staining and α-smooth muscle acting immunostaining of myofibroblasts. Gene expression of collagens, matrix modifying enzymes, cytokines and chemokines was quantified by RT-qPCR. The phenotype and the numbers of intrahepatic lymphoid and myeloid cell subsets were evaluated by flow cytometry.

Results

Both Il15-/- and Il15ra-/- mice developed markedly reduced liver fibrosis compared to wildtype control mice, as revealed by reduced collagen deposition and myofibroblast content. Il15ra-/- mice showed further reduction in collagen deposition compared to Il15-/- mice. However, Col1a1 and Col1a3 genes were similarly induced in the fibrotic livers of wildtype, Il15-/- and Il15ra-/- mice, although notable variations were observed in the expression of matrix remodeling enzymes and chemokines. As expected, Il15-/- and Il15ra-/- mice showed markedly reduced numbers of NK cells compared to wildtype mice. They also showed markedly less staining of CD45+ immune cells and CD68+ macrophages, and significantly reduced inflammatory cell infiltration into the liver, with fewer pro-inflammatory and anti-inflammatory monocyte subsets compared to wildtype mice.

Conclusion

Our findings indicate that IL-15 exerts its profibrogenic role in the liver by promoting macrophage activation and that this requires trans-presentation of IL-15 by IL-15Rα.

Effector and cytolytic function of natural killer cells in anticancer immunity

Adaptive immune cells play an important role in mounting antigen-specific antitumor immunity. The contribution of innate immune cells such as monocytes, macrophages, natural killer (NK) cells, dendritic cells, and gamma-delta T cells is well studied in cancer immunology. NK cells are innate lymphoid cells that show effector and regulatory function in a contact-dependent and contact-independent manner. The cytotoxic function of NK cells plays an important role in killing the infected and transformed host cells and controlling infection and tumor growth. However, several studies have also ascribed the role of NK cells in inducing pathophysiology in autoimmune diseases, promoting immune tolerance in the uterus, and antitumor function in the tumor microenvironment. We discuss the fundamentals of NK cell biology, its distribution in different organs, cellular and molecular interactions, and its cytotoxic and noncytotoxic functions in cancer biology. We also highlight the use of NK cell-based adoptive cellular therapy in cancer.

The role of interleukin-15 in the development and treatment of hematological malignancies

Cytokines are a vital component of the immune system that controls the activation and growth of blood cells. However, chronic overexpression of cytokines can trigger cellular events leading to malignant transformation. The cytokine interleukin-15 (IL-15) is of particular interest, which has been shown to contribute to the development and progression of various hematological malignancies. This review will provide an overview of the impact of the immunopathogenic function of IL-15 by studying its role in cell survival, proliferation, inflammation, and treatment resistance. We will also review therapeutic approaches for inhibiting IL-15 in blood cancers.

Counteracting Interleukin-15 to Elucidate Its Modes of Action in Physiology and Pathology

Interleukin (IL)-15 belongs to the common gamma-dependent cytokine family, along with IL-2, IL-4, IL-7, IL-9, and IL-21. IL-15 is crucial for the homeostasis of Natural Killer (NK) and memory CD8 T cells, and to fight against cancer progression. However, dysregulations of IL-15 expression could occur and participate in the emergence of autoimmune inflammatory diseases as well as hematological malignancies. It is therefore important to understand the different modes of action of IL-15 to decrease its harmful action in pathology without affecting its beneficial effects in the immune system. In this review, we present the different approaches used by researchers to inhibit the action of IL-15, from most broad to the most selective. Indeed, it appears that it is important to selectively target the mode of action of the cytokine rather than the cytokine itself as they are involved in numerous biological processes.

Mendelian randomization of circulating proteome identifies actionable targets in heart failure

Background

Heart failure (HF) is a prevalent cause of mortality and morbidity. The molecular drivers of HF are still largely unknown.

Results

We aimed to identify circulating proteins causally associated with HF by leveraging genome-wide genetic association data for HF including 47,309 cases and 930,014 controls. We performed two-sample Mendelian randomization (MR) with multiple cis instruments as well as network and enrichment analysis using data from blood protein quantitative trait loci (pQTL) (2,965 blood proteins) measured in 3,301 individuals. Nineteen blood proteins were causally associated with HF, were not subject to reverse causality and were enriched in ligand-receptor and glycosylation molecules. Network pathway analysis of the blood proteins showed enrichment in NF-kappa B, TGF beta, lipid in atherosclerosis and fluid shear stress. Cross-phenotype analysis of HF identified genetic overlap with cardiovascular drugs, myocardial infarction, parental longevity and low-density cholesterol. Multi-trait MR identified causal associations between HF-associated blood proteins and cardiovascular outcomes. Multivariable MR showed that association of BAG3, MIF and APOA5 with HF were mediated by the blood pressure and coronary artery disease. According to the directional effect and biological action, 7 blood proteins are targets of existing drugs or are tractable for the development of novel therapeutics. Among the pathways, sialyl Lewis x and the activin type II receptor are potential druggable candidates.

Conclusions

Integrative MR analyses of the blood proteins identified causally-associated proteins with HF and revealed pleiotropy of the blood proteome with cardiovascular risk factors. Some of the proteins or pathway related mechanisms could be targeted as novel treatment approach in HF.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08811-2.

Interleukin 15 in Cell-Based Cancer Immunotherapy

Cell-based cancer immunotherapy, such as chimeric antigen receptor (CAR) engineered T and natural killer (NK) cell therapies, has become a revolutionary new pillar in cancer treatment. Interleukin 15 (IL-15), a potent immunostimulatory cytokine that potentiates T and NK cell immune responses, has demonstrated the reliability and potency to potentially improve the therapeutic efficacy of current cell therapy. Structurally similar to interleukin 2 (IL-2), IL-15 supports the persistence of CD8⁺ memory T cells while inhibiting IL-2-induced T cell death that better maintains long-term anti-tumor immunity. In this review, we describe the biology of IL-15, studies on administrating IL-15 and/or its derivatives as immunotherapeutic agents, and IL-15-armored immune cells in adoptive cell therapy. We also discuss the advantages and challenges of incorporating IL-15 in cell-based immunotherapy and provide directions for future investigation.

Crosstalk between IL-15Rα+ tumor-associated macrophages and breast cancer cells reduces CD8+ T cell recruitment

Background

Interleukin‐15 (IL‐15) is a promising immunotherapeutic agent owing to its powerful immune‐activating effects. However, the clinical benefits of these treatments are limited. Crosstalk between tumor cells and immune cells plays an important role in immune escape and immunotherapy drug resistance. Herein, this study aimed to obtain in‐depth understanding of crosstalk in the tumor microenvironment for providing potential therapeutic strategies to prevent tumor progression.

Methods

T‐cell killing assays and co‐culture models were developed to determine the role of crosstalk between macrophages and tumor cells in breast cancer resistant to IL‐15. Western blotting, histological analysis, CRISPR‐Cas9 knockout, multi‐parameter flow cytometry, and tumor cell‐macrophage co‐injection mouse models were developed to examine the mechanism by which IL‐15Rα⁺ tumor‐associated macrophages (TAMs) regulate breast cancer cell resistance to IL‐15.

Results

We found that macrophages contributed to the resistance of tumor cells to IL‐15, and tumor cells induced macrophages to express high levels of the α subunit of the IL‐15 receptor (IL‐15Rα). Further investigation showed that IL‐15Rα⁺ TAMs reduced the protein levels of chemokine CX3C chemokine ligand 1 (CX3CL1) in tumor cells to inhibit the recruitment of CD8⁺ T cells by releasing the IL‐15/IL‐15Rα complex (IL‐15Rc). Administration of an IL‐15Rc blocking peptide markedly suppressed breast tumor growth and overcame the resistance of cancer cells to anti‐ programmed cell death protein 1 (PD‐1) antibody immunotherapy. Interestingly, Granulocyte‐macrophage colony‐stimulating factor (GMCSF) induced γ chain (γc) expression to promote tumor cell‐macrophage crosstalk, which facilitated tumor resistance to IL‐15. Additionally, we observed that the non‐transcriptional regulatory function of hypoxia inducible factor‐1alpha (HIF‐1α) was essential for IL‐15Rc to regulate CX3CL1 expression in tumor cells.

Conclusions

The IL‐15Rc‐HIF‐1α‐CX3CL1 signaling pathway serves as a crosstalk between macrophages and tumor cells in the tumor microenvironment of breast cancer. Targeting this pathway may provide a potential therapeutic strategy for enhancing the efficacy of cancer immunotherapy.

Selective Targeting of IL-15Rα Is Sufficient to Reduce Inflammation

Cytokines are crucial molecules for maintaining the proper functioning of the immune system. Nevertheless, a dysregulation of cytokine expression could be involved in the pathogenesis of autoimmune diseases. Interleukin (IL)-15 is a key factor for natural killer cells (NK) and CD8 T cells homeostasis, necessary to fight cancer and infections but could also be considered as a pro-inflammatory cytokine involved in autoimmune inflammatory disease, including rheumatoid arthritis, psoriasis, along with tumor necrosis factor alpha (TNF-α), IL-6, and IL-1β. The molecular mechanisms by which IL-15 exerts its inflammatory function in these diseases are still unclear. In this study, we generated an IL-15-derived molecule called NANTIL-15 (New ANTagonist of IL-15), designed to selectively inhibit the action of IL-15 through the high-affinity trimeric IL-15Rα/IL-2Rβ/γc receptor while leaving IL-15 signaling through the dimeric IL-2Rβ/γc receptor unaffected. Administrating of NANTIL-15 in healthy mice did not affect the IL-15-dependent cell populations such as NK and CD8 T cells. In contrast, we found that NANTIL-15 efficiently reduced signs of inflammation in a collagen-induced arthritis model. These observations demonstrate that the inflammatory properties of IL-15 are linked to its action through the trimeric IL-15Rα/IL-2Rβ/γc receptor, highlighting the interest of selectively targeting this receptor.

Engineering interferons and interleukins for cancer immunotherapy

Cytokines are a class of potent immunoregulatory proteins that are secreted in response to various stimuli and act locally to regulate many aspects of human physiology and disease. Cytokines play important roles in cancer initiation, progression, and elimination, and thus, there is a long clinical history associated with the use of recombinant cytokines to treat cancer. However, the use of cytokines as therapeutics has been limited by cytokine pleiotropy, complex biology, poor drug-like properties, and severe dose-limiting toxicities. Nevertheless, cytokines are crucial mediators of innate and adaptive antitumor immunity and have the potential to enhance immunotherapeutic approaches to treat cancer. Development of immune checkpoint inhibitors and combination immunotherapies has reinvigorated interest in cytokines as therapeutics, and a variety of engineering approaches are emerging to improve the safety and effectiveness of cytokine immunotherapy. In this review we highlight recent advances in cytokine biology and engineering for cancer immunotherapy.

Soluble forms of cytokine and growth factor receptors: Mechanisms of generation and modes of action in the regulation of local and systemic inflammation

Cytokine and growth factor receptors are usually transmembrane proteins but they can also exist in soluble forms, either through cleavage and release of their ligand-binding extracellular domain, or through secretion of a soluble isoform. As an extension of this concept, transmembrane receptors on exosomes released into the circulation may act similarly to circulating soluble receptors. These soluble receptors add to the complexity of cytokine and growth factor signalling: they can function as decoy receptor that compete for ligand binding with their respective membrane-bound forms thereby attenuating signalling, or stabilize their ligands, or activate additional signalling events through interactions with other cell-surface proteins. Their soluble nature allows for a functional role away from the production sites, in remote cell types and organs. Accumulating evidence demonstrates that soluble receptors participate in the regulation and orchestration of various key cellular processes, particularly inflammatory responses. In this review, we will discuss release mechanisms of soluble cytokine and growth factor receptors, their mechanisms of action, as well as strategies for targeting their pathways in disease.

IL-15Rα-Independent IL-15 Signaling in Non-NK Cell-Derived IFNγ Driven Control of Listeria monocytogenes

Interleukin-15, produced by hematopoietic and parenchymal cells, maintains immune cell homeostasis and facilitates activation of lymphoid and myeloid cell subsets. IL-15 interacts with the ligand-binding receptor chain IL-15Rα during biosynthesis, and the IL-15:IL-15Rα complex is trans-presented to responder cells that express the IL-2/15Rβγ_c complex to initiate signaling. IL-15-deficient and IL-15Rα-deficient mice display similar alterations in immune cell subsets. Thus, the trimeric IL-15Rαβγ_c complex is considered the functional IL-15 receptor. However, studies on the pathogenic role of IL-15 in inflammatory and autoimmune diseases indicate that IL-15 can signal independently of IL-15Rα via the IL-15Rβγ_c dimer. Here, we compared the ability of mice lacking IL-15 (no signaling) or IL-15Rα (partial/distinct signaling) to control Listeria monocytogenes infection. We show that IL-15-deficient mice succumb to infection whereas IL-15Rα-deficient mice clear the pathogen as efficiently as wildtype mice. IL-15-deficient macrophages did not show any defect in bacterial uptake or iNOS expression in vitro. In vivo, IL-15 deficiency impaired the accumulation of inflammatory monocytes in infected spleens without affecting chemokine and pro-inflammatory cytokine production. The inability of IL-15-deficient mice to clear L. monocytogenes results from impaired early IFNγ production, which was not affected in IL-15Rα-deficient mice. Administration of IFNγ partially enabled IL-15-deficient mice to control the infection. Bone marrow chimeras revealed that IL-15 needed for early bacterial control can originate from both hematopoietic and non-hematopoietic cells. Overall, our findings indicate that IL-15-dependent IL-15Rα-independent signaling via the IL-15Rβγ_c dimeric complex is necessary and sufficient for the induction of IFNγ from sources other than NK/NKT cells to control bacterial pathogens.

IL-15 Trans-Presentation Is an Autonomous, Antigen-Independent Process

IL-15 plays a pivotal role in the long-term survival of T cells and immunological memory. Its receptor consists of three subunits (IL-15Rα, IL-2/15Rβ, and γ_c). IL-15 functions mainly via trans-presentation (TP), during which an APC expressing IL-15 bound to IL-15Rα presents the ligand to the βγ_c receptor-heterodimer on a neighboring T/NK cell. To date, no direct biophysical evidence for the intercellular assembly of the IL-15R heterotrimer exists. Ag presentation (AP), the initial step of T cell activation, is also based on APC-T cell interaction. We were compelled to ask whether AP has any effect on IL-15 TP or whether they are independent processes. In our human Raji B cell-Jurkat T cell model system, we monitored inter-/intracellular protein interactions upon formation of IL-15 TP and AP receptor complexes by Förster resonance energy transfer measurements. We detected enrichment of IL-15Rα and IL-2/15Rβ at the synapse and positive Förster resonance energy transfer efficiency if Raji cells were pretreated with IL-15, giving direct biophysical evidence for IL-15 TP. IL-15Rα and MHC class II interacted and translocated jointly to the immunological synapse when either ligand was present, whereas IL-2/15Rβ and CD3 moved independently of each other. IL-15 TP initiated STAT5 phosphorylation in Jurkat cells, which was not further enhanced by AP. Conversely, IL-15 treatment slightly attenuated Ag-induced phosphorylation of the CD3ζ chain. Our studies prove that in our model system, IL-15 TP and AP can occur independently, and although AP enhances IL-15R assembly, it has no significant effect on IL-15 signaling during TP. Thus, IL-15 TP can be considered an autonomous, Ag-independent process.

Interleukin-15 and cancer: some solved and many unsolved questions

Soluble interleukin (IL)-15 exists under two forms: as monomer (sIL-15) or as heterodimeric complex in association with sIL-15Rα (sIL-15/IL-15Rα). Both forms have been successfully tested in experimental tumor murine models and are currently undergoing investigation in phase I/II clinical trials. Despite more than 20 years research on IL-15, some controversial issues remain to be addressed. A first point concerns the detection of the sIL-15/IL-15Rα in plasma of healthy donors or patients with cancer and its biological significance. The second and third unsolved question regards the protumorigenic role of the IL-15/IL-15Rα complex in human cancer and the detrimental immunological consequences associated to prolonged exposure of natural killer (NK) cells to both forms of soluble IL-15, respectively. Data suggest that in vivo prolonged or repeated exposure to monomeric sIL-15 or the soluble complex may lead to NK hypo-responsiveness through the expansion of the CD8⁺/CD44⁺ T cell subset that would suppress NK cell functions. In vitro experiments indicate that soluble complex and monomeric IL-15 may cause NK hyporesponsiveness through a direct effect caused by their prolonged stimulation, suggesting that this mechanism could also be effective in vivo. Therefore, a better knowledge of IL-15 and a more appropriate use of both its soluble forms, in terms of concentrations and time of exposure, are essential in order to improve their therapeutic use. In cancer, the overproduction of sIL-15/IL-15Rα could represent a novel mechanism of immune escape. The soluble complex may act as a decoy cytokine unable to efficiently foster NK cells, or could induce NK hyporesponsiveness through an excessive and prolonged stimulation depending on the type of IL-15Rα isoforms associated. All these unsolved questions are not merely limited to the knowledge of IL-15 pathophysiology, but are crucial also for the therapeutic use of this cytokine. Therefore, in this review, we will discuss key unanswered issues on the heterogeneity and biological significance of IL-15 isoforms, analyzing both their cancer-related biological functions and their therapeutic implications.

NKTR-255, a polymer-conjugated IL-15 with unique mechanisms of action on T and natural killer cells

NKTR-255 is a novel polyethylene glycol (PEG)-conjugate of recombinant human IL-15 (rhIL-15) being examined as a potential cancer immunotherapeutic. Since IL-15 responses can be mediated by trans- or cis-presentation via IL-15Rα or soluble IL-15/IL-15Rα complexes, we investigated the role of IL-15Rα in driving NKTR-255 responses using defined naïve and memory ovalbumin-specific CD8 T cells (OT-I) CD8 T and NK cells in mice. NKTR-255 induced a 2.5 and 2.0-fold expansion of CD8 T and NK cells, respectively in WT mice. In adoptive transfer studies, proliferation of naïve and memory Wt OT-I T cells in response to NKTR-255 was not impaired in IL-15Rα-/- mice, suggesting trans-presentation was not utilized by NKTR-255. Interestingly, naïve IL-15Rα-/- OT-I cells had deficient responses to NKTR-255 while memory IL-15Rα-/- OT-I cell responses were partially impaired, suggesting that naive CD8 T cells are more dependent on cis-presentation of NKTR-255 than memory CD8 T cells. In bone marrow chimeras studies, IL-15Rα-/- and WT NK cells present in WT recipients had similar responses to NKTR-255, suggesting that cis-presentation is not utilized by NK cells. NKTR-255 could form soluble complexes with IL-15Rα; binding to murine IL-15Rα generated superagonists that preferentially stimulated NK cells showing that conversion to IL-15Rβ agonist biases the response towards NK cells. These findings highlight the ability of NKTR-255 to utilize IL-15Rα for cis-presentation and act as an IL-15Rαβ agonist on CD8 T cells.

Interleukin-15 in autoimmunity

Interleukin-15 (IL-15) is a member of the IL-2 family of cytokines, which use receptor complexes containing the common gamma (γ_c) chain for signaling. IL-15 plays important roles in innate and adaptative immune responses and is implicated in the pathogenesis of several immune diseases. The IL-15 receptor consists of 3 subunits namely, the ligand-binding IL-15Rα chain, the β chain (also used by IL-2) and the γ_c chain. IL-15 uses a unique signaling pathway whereby IL-15 associates with IL-15Rα during biosynthesis, and this complex is 'trans-presented' to responder cells that expresses the IL-2/15Rβγ_c receptor complex. IL-15 is subject to post-transcriptional and post-translational regulation, and evidence also suggests that IL-15 cis-signaling can occur under certain conditions. IL-15 has been implicated in the pathology of various autoimmune diseases such as rheumatoid arthritis, autoimmune diabetes, inflammatory bowel disease, coeliac disease and psoriasis. Studies with pre-clinical models have shown the beneficial effects of targeting IL-15 signaling in autoimmunity. Unlike therapies targeting other cytokines, anti-IL-15 therapies have not yet been successful in humans. We discuss the complexities of IL-15 signaling in autoimmunity and explore potential immunotherapeutic approaches to target the IL-15 signaling pathway.

Transcriptional Regulation of Natural Killer Cell Development and Functions

Natural killer (NK) cells are the major lymphocyte subset of the innate immune system. Their ability to mediate anti-tumor cytotoxicity and produce cytokines is well-established. However, the molecular mechanisms associated with the development of human or murine NK cells are not fully understood. Knowledge is being gained about the environmental cues, the receptors that sense the cues, signaling pathways, and the transcriptional programs responsible for the development of NK cells. Specifically, a complex network of transcription factors (TFs) following microenvironmental stimuli coordinate the development and maturation of NK cells. Multiple TFs are involved in the development of NK cells in a stage-specific manner. In this review, we summarize the recent advances in the understandings of TFs involved in the regulation of NK cell development, maturation, and effector function, in the aspects of their mechanisms, potential targets, and functions.

Biological functions and therapeutic opportunities of soluble cytokine receptors

Cytokines control the immune system by regulating the proliferation, differentiation and function of immune cells. They activate their target cells through binding to specific receptors, which either are transmembrane proteins or attached to the cell-surface via a GPI-anchor. Different tissues and individual cell types have unique expression profiles of cytokine receptors, and consequently this expression pattern dictates to which cytokines a given cell can respond. Furthermore, soluble variants of several cytokine receptors exist, which are generated by different molecular mechanisms, namely differential mRNA splicing, proteolytic cleavage of the membrane-tethered precursors, and release on extracellular vesicles. These soluble receptors shape the function of cytokines in different ways: they can serve as antagonistic decoy receptors which compete with their membrane-bound counterparts for the ligand, or they can form functional receptor/cytokine complexes which act as agonists and can even activate cells that would usually not respond to the ligand alone. In this review, we focus on the IL-2 and IL-6 families of cytokines and the so-called Th2 cytokines. We summarize for each cytokine which soluble receptors exist, were they originate from, how they are generated, and what their biological functions are. Furthermore, we give an outlook on how these soluble receptors can be exploited for therapeutic purposes.

Memory T cells: strategies for optimizing tumor immunotherapy

Several studies have demonstrated that memory T cells including stem cell memory (Tscm) T cells and central memory (Tcm) T cells show superior persistence and antitumor immunity compared with effector memory T (Tem) cells and effector T (Teff) cells. Furthermore, the Tcm/Teff ratio has been reported to be a predictive biomarker of immune responses against some tumors. Thus, a system-level understanding of the mechanisms underlying the differentiation of effector and memory T cells is of increasing importance for developing immunological strategies against various tumors. This review focuses on recent advances in efficacy against tumors, the origin, formation mechanisms of memory T cells, and the role of the gut microbiota in memory T cell formation. Furthermore, we summarize strategies to generate memory T cells in (ex) vivo that, might be applicable in clinical practice.

The IL-15 / sIL-15Rα complex modulates immunity without effect on asthma features in mouse

BACKGROUND

Interleukin 15 (IL-15) is a growth and modulating factor for B, T lymphocytes and natural killer cells (NK). Its action on innate and adaptive immunity is modulated by its alpha chain receptor (IL-15Rα). The IL-15/sIL-15Rα complex (IL-15Cx) increases the bioavailability and activity of the cytokine in vivo. IL-15Cx has been used in diseases to dampen IL-15 inflammation by the use of soluble IL-15Ralpha specificity. Here, we aim to evaluate the interest of IL-15Cx in a mouse model of asthma.

METHODS

Using a mouse model of asthma consisting in percutaneous sensitization and intranasal challenge with total house dust mite extract, we evaluated the effect of IL-15Cx injected intraperitoneally four times after a first nasal challenge. Respiratory function was assessed by the technique of forced oscillations (Flexivent®). The effect on bronchial remodeling was evaluated by lung histology. The inflammatory status was analyzed by flow cytometry.

RESULTS

We observed that the IL-15Cx modulates lung and systemic inflammation by increasing NK cells, CD8⁺ memory T cells and regulatory cells. However, IL-15Cx displays no effect on bronchial hyperreactivity, bronchial remodeling nor cellular bronchial infiltrate, but limits the secretion of bronchial mucus and modulates only inflammatory response in a HDM-allergic asthma murine model.

CONCLUSIONS

IL-15Cx has a limited effect on immune response in asthma and has no effect on lung function in mice. Thus, it limits its therapeutic potential but might suggest a combinatory potential with other therapeutics.

Trans-endocytosis of intact IL-15Rα-IL-15 complex from presenting cells into NK cells favors signaling for proliferation

Interleukin 15 (IL-15) is an essential cytokine for the survival and proliferation of natural killer (NK) cells. IL-15 activates signaling by the β and common γ (γ_c) chain heterodimer of the IL-2 receptor through trans-presentation by cells expressing IL-15 bound to the α chain of the IL-15 receptor (IL-15Rα). We show here that membrane-associated IL-15Rα-IL-15 complexes are transferred from presenting cells to NK cells through trans-endocytosis and contribute to the phosphorylation of ribosomal protein S6 and NK cell proliferation. NK cell interaction with soluble or surface-bound IL-15Rα-IL-15 complex resulted in Stat5 phosphorylation and NK cell survival at a concentration or density of the complex much lower than required to stimulate S6 phosphorylation. Despite this efficient response, Stat5 phosphorylation was reduced after inhibition of metalloprotease-induced IL-15Rα-IL-15 shedding from trans-presenting cells, whereas S6 phosphorylation was unaffected. Conversely, inhibition of trans-endocytosis by silencing of the small GTPase TC21 or expression of a dominant-negative TC21 reduced S6 phosphorylation but not Stat5 phosphorylation. Thus, trans-endocytosis of membrane-associated IL-15Rα-IL-15 provides a mode of regulating NK cells that is not afforded to IL-2 and is distinct from activation by soluble IL-15. These results may explain the strict IL-15 dependence of NK cells and illustrate how the cellular compartment in which receptor-ligand interaction occurs can influence functional outcome.

IL-15Rα membrane anchorage in either cis or trans is required for stabilization of IL-15 and optimal signaling

Interleukin (IL)-15 plays an important role in the communication between immune cells. It delivers its signal through different modes involving three receptor chains: IL-15Rα, IL-2Rβ and IL-2Rγc. The combination of the different chains result in the formation of IL-15Rα/IL-2Rβ/γc trimeric or IL-2Rβ/γc dimeric receptors. In this study, we have investigated the role of the IL-15Rα chain in stabilizing the cytokine in the IL-2Rβ/γc dimeric receptor. By analyzing the key amino acid residues of IL-15 facing IL-2Rβ, we provide evidence of differential interfaces in the presence or in the absence of membrane-anchored IL-15Rα. Moreover, we found that the anchorage of IL-15Rα to the cell surface regardless its mode of presentation - i.e. cis or trans - is crucial for complete signaling. These observations show how the cells can finely modulate the intensity of cytokine signaling through the quality and the level of expression of the receptor chains.

Interleukin-15 as a myokine: mechanistic insight into its effect on skeletal muscle metabolism

Interleukin (IL)-15 is a cytokine with important immunological functions. It is highly expressed in skeletal muscle and is believed to be a myokine, a hypothesis supported by the rapid increase in circulating levels of IL-15 in response to exercise. Treatment with high doses of IL-15 results in metabolic adaptations such as improved insulin sensitivity and whole-body fatty acid oxidation and protection from high-fat-diet-induced obesity and insulin resistance. IL-15 secreted by contracting muscle may therefore act as an endocrine factor to improve adiposity and energy metabolism in different tissues. Most studies have used supraphysiological doses of IL-15 that do not represent circulating IL-15 in response to exercise. However, evidence shows that IL-15 levels are higher in muscle interstitium and that IL-15 might improve muscle glucose homeostasis and oxidative metabolism in an autocrine/paracrine manner. Nevertheless, how IL-15 signals in skeletal muscle to improve muscle energy metabolism is not understood completely, especially because the absence of the α subunit of the IL-15 receptor (IL-15Rα) results in a phenotype similar to that of overexpressing/oversecreting IL-15 in mice. In this article, we review the literature to propose a model for the regulation of IL-15 by the soluble form of IL-15Rα to explain why some findings in the literature seem, at first glance, to be contradictory.

Cytokine-targeted therapy for the management of solid organ transplant recipients

INTRODUCTION

The number of solid organ transplants completed annually continues to trend upwards each year. Despite this, maintenance immunosuppression available on the market has remained relatively stagnant. Standard triple immunosuppression, composed typically of tacrolimus, mycophenolate, and steroids, lead to many side effects that limit the use of these medications. Tacrolimus, specifically, causes nephrotoxicity that can lead to renal dysfunction requiring a kidney transplant down the road. Alternative therapies for the management of immunosuppression need to be identified to try to mitigate these adverse effects. BODY: Cytokines are responsible for facilitating T cell differentiation and lead to the activation of inflammatory mediators that can contribute to graft damage and ultimately rejection. IL-4, IL-6, IL-12/23, and IL-15 are attractive targets for medications to try to ameliorate graft rejection. Various cytokine-targeted medications are currently available on the market for the treatment of inflammatory and autoimmune conditions such as rheumatoid arthritis, psoriatic arthritis, Crohn's, and multiple sclerosis.

CONCLUSION

This article reviews cytokine involvement in alloimmunity and the potential role cytokine-targeted therapy may play in prevention of allograft rejection in solid organ transplant recipients.

Natural Killer Cells: Development, Maturation, and Clinical Utilization

Natural killer (NK) cells are the predominant innate lymphocyte subsets that mediate anti-tumor and anti-viral responses, and therefore possess promising clinical utilization. NK cells do not express polymorphic clonotypic receptors and utilize inhibitory receptors (killer immunoglobulin-like receptor and Ly49) to develop, mature, and recognize “self” from “non-self.” The essential roles of common gamma cytokines such as interleukin (IL)-2, IL-7, and IL-15 in the commitment and development of NK cells are well established. However, the critical functions of pro-inflammatory cytokines IL-12, IL-18, IL-27, and IL-35 in the transcriptional-priming of NK cells are only starting to emerge. Recent studies have highlighted multiple shared characteristics between NK cells the adaptive immune lymphocytes. NK cells utilize unique signaling pathways that offer exclusive ways to genetically manipulate to improve their effector functions. Here, we summarize the recent advances made in the understanding of how NK cells develop, mature, and their potential translational use in the clinic.

IL-2 and Beyond in Cancer Immunotherapy

The development of the T- and natural killer (NK) cell growth factor IL-2 has been a sentinel force ushering in the era of immunotherapy in cancer. With the advent of clinical grade recombinant IL-2 in the mid-1980s, oncologists could for the first time directly manipulate lymphocyte populations with systemic therapy. By itself, recombinant IL-2 can induce clinical responses in up to 15% of patients with metastatic cancer or renal cell carcinoma. When administered with adoptively transferred tumor-reactive lymphocytes, IL-2 promotes T cell engraftment and response rates of up to 50% in metastatic melanoma patients. Importantly, these IL-2-driven responses can yield complete and durable responses in a subset of patients. However, the use of IL-2 is limited by toxicity and concern of the expansion of T regulatory cells. To overcome these limitations and improve response rates, other T cell growth factors, including IL-15 and modified forms of IL-2, are in clinical development. Administering T cell growth factors in combination with other agents, such as immune checkpoint pathway inhibitors, may also improve efficacy. In this study, we review the development of T- and NK cell growth factors and highlight current combinatorial approaches based on these reagents.

Interleukin-6 Family Cytokines

The interleukin (IL)-6 family cytokines is a group of cytokines consisting of IL-6, IL-11, ciliary neurotrophic factor (CNTF), leukemia inhibitory factor (LIF), oncostatin M (OSM), cardiotrophin 1 (CT-1), cardiotrophin-like cytokine (CLC), and IL-27. They are grouped into one family because the receptor complex of each cytokine contains two (IL-6 and IL-11) or one molecule (all others cytokines) of the signaling receptor subunit gp130. IL-6 family cytokines have overlapping but also distinct biologic activities and are involved among others in the regulation of the hepatic acute phase reaction, in B-cell stimulation, in the regulation of the balance between regulatory and effector T cells, in metabolic regulation, and in many neural functions. Blockade of IL-6 family cytokines has been shown to be beneficial in autoimmune diseases, but bacterial infections and metabolic side effects have been observed. Recent advances in cytokine blockade might help to minimize such side effects during therapeutic blockade.

Immunobiology of the IL-15/IL-15Rα complex as an antitumor and antiviral agent

Interleukin (IL)-15 is essential for natural killer (NK), NKT and memory (m) CD8⁺ T cell development and function, and is currently under investigation as an immunotherapeutic agent for the treatment of cancer. Recently, the creation of IL-15 superagonist by complexing IL-15 and its high affinity receptor alpha (IL-15 Rα) in solution, inspired by the natural trans-presentation of IL-15, advances the potential of IL-15-based tumor immunotherapy. IL-15 superagonist shows promising advantages over monomeric IL-15 such as sustaining high circulating concentrations due to prolonged half-life and more potently stimulating NK and CD8⁺ T effector lymphocytes. So far, there are three different forms of recombinant IL-15 superagonist fusion protein based on configurational modifications. Gene therapy using engineered cells co-expressing IL-15/IL-15 Rα complex for cancer treatment is also emerging. All forms have demonstrated efficacy in causing tumor regression in animal studies, which provides strong rationale for advancing IL-15 superagonist through clinical trials. To date, there are fourteen phase I/II IL-15 superagonist trials in cancer patients and one phase I trial in HIV patients. Information generated by ongoing trials regarding the toxicity and efficacy of IL-15 superagonist is awaited. Finally, we elaborate on immunotoxicity caused by IL-15 superagonist in preclinical studies and discuss important safety considerations.

CD4 effector T cell differentiation is controlled by IL-15 that is expressed and presented in trans

T cells are both producers and consumers of cytokines, and autocrine cytokine signaling plays a critical role in T cell immunity. IL-15 is a homeostatic cytokine for T cells that also controls inflammatory immune responses. An autocrine role of T cell-derived IL-15, however, remains unclear. Here we examined IL-15 expression and signaling upon effector T cell differentiation in mice, and, surprisingly, found that CD4 T cells did not express IL-15. CD4 T cells lacked Il15 gene reporter activity, did not contain IL-15 transcripts, and did not produce IL-15Rα, the proprietary IL-15 receptor required for IL-15 trans-presentation. Moreover, IL-15 failed to inhibit Th17 cell differentiation and failed to generate Foxp3⁺ Treg cells in vitro. IL-2, which utilizes the same IL-2Rβ/γc receptor complex, however, successfully did so. Exogenous IL-15 only exerted bioactivity and controlled T cell differentiation when it was trans-presented by IL-15Rα. Consequently, IL-15Rα-bound IL-15, but not free IL-15, suppressed Th17 cell differentiation and induced Treg cell generation. Collectively, these results reveal the absence of an IL-15 autocrine loop in CD4 T cells and strongly suggest that IL-15 trans-presentation by non-CD4 T cells is the primary mechanism via which IL-15 controls CD4 effector T cell differentiation.

Skeletal muscle IL-15/IL-15Rα and myofibrillar protein synthesis after resistance exercise

In vitro and in vivo studies described the myokine IL-15 and its receptor IL-15Rα as anabolic/anti-atrophy agents, however, the protein expression of IL-15Rα has not been measured in human skeletal muscle and data regarding IL-15 expression remain inconclusive. The purpose of the study was to determine serum and skeletal muscle IL-15 and IL-15Rα responses to resistance exercise session and to analyze their association with myofibrillar protein synthesis (MPS). Fourteen participants performed a bilateral leg resistance exercise composed of four sets of leg press and four sets of knee extension at 75% 1RM to task failure. Muscle biopsies were obtained at rest, 0, 4 and 24 hours post-exercise and blood samples at rest, mid-exercise, 0, 0.3, 1, 2, 4 and 24 hours post-exercise. Serum IL-15 was increased by ~5.3-fold immediately post-exercise, while serum IL-15Rα decreased ~75% over 1 hour post-exercise (P<.001). Skeletal muscle IL-15Rα mRNA and protein expression were increased at 4 hours post-exercise by ~2-fold (P<.001) and ~1.3-fold above rest (P=.020), respectively. At 24 hours post-exercise, IL-15 (P=.003) and IL-15Rα mRNAs increased by ~2-fold (P=.002). Myofibrillar fractional synthetic rate between 0-4 hours was associated with IL-15Rα mRNA at rest (r=.662, P=.019), 4 hours (r=.612, P=.029), and 24 hours post-exercise (r=.627, P=.029). Finally, the muscle IL-15Rα protein up-regulation was related to Leg press 1RM (r=.688, P=.003) and total weight lifted (r=.628, P=.009). In conclusion, IL-15/IL-15Rα signaling pathway is activated in skeletal muscle in response to a session of resistance exercise.

SheddomeDB: the ectodomain shedding database for membrane-bound shed markers

BACKGROUND

A number of membrane-anchored proteins are known to be released from cell surface via ectodomain shedding. The cleavage and release of membrane proteins has been shown to modulate various cellular processes and disease pathologies. Numerous studies revealed that cell membrane molecules of diverse functional groups are subjected to proteolytic cleavage, and the released soluble form of proteins may modulate various signaling processes. Therefore, in addition to the secreted protein markers that undergo secretion through the secretory pathway, the shed membrane proteins may comprise an additional resource of noninvasive and accessible biomarkers. In this context, identifying the membrane-bound proteins that will be shed has become important in the discovery of clinically noninvasive biomarkers. Nevertheless, a data repository for biological and clinical researchers to review the shedding information, which is experimentally validated, for membrane-bound protein shed markers is still lacking.

RESULTS

In this study, the database SheddomeDB was developed to integrate publicly available data of the shed membrane proteins. A comprehensive literature survey was performed to collect the membrane proteins that were verified to be cleaved or released in the supernatant by immunological-based validation experiments. From 436 studies on shedding, 401 validated shed membrane proteins were included, among which 199 shed membrane proteins have not been annotated or validated yet by existing cleavage databases. SheddomeDB attempted to provide a comprehensive shedding report, including the regulation of shedding machinery and the related function or diseases involved in the shedding events. In addition, our published tool ShedP was embedded into SheddomeDB to support researchers for predicting the shedding event on unknown or unrecorded membrane proteins.

CONCLUSIONS

To the best of our knowledge, SheddomeDB is the first database for the identification of experimentally validated shed membrane proteins and currently may provide the most number of membrane proteins for reviewing the shedding information. The database included membrane-bound shed markers associated with numerous cellular processes and diseases, and some of these markers are potential novel markers because they are not annotated or validated yet in other databases. SheddomeDB may provide a useful resource for discovering membrane-bound shed markers. The interactive web of SheddomeDB is publicly available at http://bal.ym.edu.tw/SheddomeDB/ .

Characterization of Receptor-Associated Protein Complex Assembly in Interleukin (IL)-2- and IL-15-Activated T-Cell Lines

It remains a paradox that IL-2 and IL-15 can differentially modulate the immune response using the same signaling receptors. We have previously dissected the phosphotyrosine-driven signaling cascades triggered by both cytokines in Kit225 T-cells, unveiling subtle differences that may contribute to their functional dichotomy. In this study, we aimed to decipher the receptor complex assembly in IL-2- and IL-15-activated T-lymphocytes that is highly orchestrated by site-specific phosphorylation events. Comparing the cytokine-induced interactome of the interleukin receptor beta and gamma subunits shared by the two cytokines, we defined the components of the early IL-2 and IL-15 receptor-associated complex discovering novel constituents. Additionally, phosphopeptide-directed analysis allowed us to detect several cytokine-dependent and -independent phosphorylation events within the activated receptor complex including novel phosphorylated sites located in the cytoplasmic region of IL-2 receptor β subunit (IL-2Rβ). We proved that the distinct phosphorylations induced by the cytokines serve for recruiting different types of effectors to the initial receptor/ligand complex. Overall, our study sheds new light into the initial molecular events triggered by IL-2 and IL-15 and constitutes a further step toward a better understanding of the early signaling aspects of the two closely related cytokines in T-lymphocytes.

Mechanisms of immunomodulation by mammalian and viral decoy receptors: insights from structures

Immune responses are regulated by effector cytokines and chemokines that signal through cell surface receptors. Mammalian decoy receptors - which are typically soluble or inactive versions of cell surface receptors or soluble protein modules termed binding proteins - modulate and antagonize signalling by canonical effector-receptor complexes. Viruses have developed a diverse array of molecular decoys to evade host immune responses; these include viral homologues of host cytokines, chemokines and chemokine receptors; variants of host receptors with new functions; and novel decoy receptors that do not have host counterparts. Over the past decade, the number of known mammalian and viral decoy receptors has increased considerably, yet a comprehensive curation of the corresponding structure-mechanism relationships has not been carried out. In this Review, we provide a comprehensive resource on this topic with a view to better understanding the roles and evolutionary relationships of mammalian and viral decoy receptors, and the opportunities for leveraging their therapeutic potential.

Deficiency of Interleukin-15 Confers Resistance to Obesity by Diminishing Inflammation and Enhancing the Thermogenic Function of Adipose Tissues

OBJECTIVE

IL-15 is an inflammatory cytokine secreted by many cell types. IL-15 is also produced during physical exercise by skeletal muscle and has been reported to reduce weight gain in mice. Contrarily, our findings on IL-15 knockout (KO) mice indicate that IL-15 promotes obesity. The aim of this study is to investigate the mechanisms underlying the pro-obesity role of IL-15 in adipose tissues.

METHODS

Control and IL-15 KO mice were maintained on high fat diet (HFD) or normal control diet. After 16 weeks, body weight, adipose tissue and skeletal mass, serum lipid levels and gene/protein expression in the adipose tissues were evaluated. The effect of IL-15 on thermogenesis and oxygen consumption was also studied in primary cultures of adipocytes differentiated from mouse preadipocyte and human stem cells.

RESULTS

Our results show that IL-15 deficiency prevents diet-induced weight gain and accumulation of lipids in visceral and subcutaneous white and brown adipose tissues. Gene expression analysis also revealed elevated expression of genes associated with adaptive thermogenesis in the brown and subcutaneous adipose tissues of IL-15 KO mice. Accordingly, oxygen consumption was increased in the brown adipocytes from IL-15 KO mice. In addition, IL-15 KO mice showed decreased expression of pro-inflammatory mediators in their adipose tissues.

CONCLUSIONS

Absence of IL-15 results in decreased accumulation of fat in the white adipose tissues and increased lipid utilization via adaptive thermogenesis. IL-15 also promotes inflammation in adipose tissues that could sustain chronic inflammation leading to obesity-associated metabolic syndrome.

IL-2, IL-7, and IL-15: Multistage regulators of CD4(+) T helper cell differentiation

Cytokines represent a class of environmental factors that are critical drivers of immune cell development. Cytokines of the common gamma-chain family, including interleukin (IL)-2, IL-7, and IL-15, have been the subject of intense experimental scrutiny and have well-defined roles as regulators of diverse immune cell types including CD4(+) T helper cells. Because of their pleiotropic effects on T-cell development and function, researchers and clinicians have attempted to harness the capabilities of these cytokines for therapeutic benefit. In this review, we summarize the recent progress in our understanding of the molecular mechanisms underlying the effects of these cytokines on CD4(+) T cell development and briefly discuss how these immunomodulatory cytokines are being used in efforts to treat human disease.

Free IL-15 Is More Abundant Than IL-15 Complexed With Soluble IL-15 Receptor-α in Murine Serum: Implications for the Mechanism of IL-15 Secretion

IL-15 is a cytokine that is part of the innate immune system, as well as a proposed myokine released from skeletal muscle during physical exercise that mediates many of the positive physiological effects of exercise. Many of the immune functions of IL-15 are mediated by juxtacrine signaling via externalized IL-15 bound to membrane-associated IL-15 receptor-α (IL-15Rα). Serum and plasma samples also contain measurable concentrations of IL-15, believed to arise from proteolytic cleavage of membrane-associated IL-15/IL-15Rα complexes to generate soluble IL-15/IL-15Rα species. Here, we validate commercial assays that can distinguish the free form of IL-15 and IL-15/IL-15Rα complexes. These assays showed that most (86%) IL-15 in mouse serum resides in the free state, with a minor proportion (14%) residing in complex with IL-15Rα. Given the much shorter half-life of free IL-15 compared with IL-15/IL-15Rα complexes, these findings cast doubt on the currently accepted model for IL-15 secretion from cleavage of membrane-bound IL-15/IL-15Rα and suggest that IL-15 is released as a free molecule by an unknown mechanism.

Transmembrane-Bound IL-15-Promoted Epithelial-Mesenchymal Transition in Renal Cancer Cells Requires the Src-Dependent Akt/GSK-3β/β-Catenin Pathway

Intrarenal interleukin-15 (IL-15) plays a major role controlling epithelial survival and polarization both in physiological and pathologic conditions. Herein, we confirmed that human renal cell carcinomas (RCCs) express a membrane-bound IL-15 isoform displaying an unusual molecular weight of 27 kDa. Its stimulation with soluble IL-15 receptor α chain (s-IL-15Rα) triggers epithelial-mesenchymal transition (EMT) process as shown by the down-regulation of E-cadherin and zona occludens 1 and the up-regulation of vimentin and N-cadherin and promotes the migratory and invasive properties of RCC. S-IL-15Rα treatment triggered the Src/PI3K/Akt/GSK-3β pathway and promoted β-catenin nuclei translocation. Deactivation of this pathway by using Src-specific inhibitor PP2, PI3K inhibitor LY294002, and AKT inhibitor MK2206 hampered β-catenin nuclei translocation and suppressed EMT, migration, and invasion of RCC. S-IL-15Rα treatment also enhanced Src-dependent phosphorylation of focal adhesion kinase (FAK) and extracellular signal-regulated kinase (Erk1/2). FAK knockdown significantly decreased the migration and invasion of RCC, which suggest that Src-FAK signaling was involved in s-IL-15Rα-favored migration and invasion of RCC. At the same time, inhibitors of Erk1/2 also significantly decreased the migration and invasion of RCC but could not reverse s-IL-15Rα-induced EMT. Taken together, our results reveal that Src-dependent PI3K/Akt/GSK3b/β-catenin pathway is required for s-IL-15Ra-dependent induction of EMT in RCC, while Src-FAK and Src-Erk1/2 signaling were involved in s-IL-15Rα-promoted migration and invasion properties of RCC. Our study provides a better understanding of IL-15 signaling in RCC tumor progression, which may lead to novel targeted therapies and provide some suggestions when using IL-15 in clinic.

Trans-presentation of interleukin-15 by interleukin-15 receptor alpha is dispensable for the pathogenesis of autoimmune type 1 diabetes

Interleukin-15 (IL-15) is a pro-inflammatory cytokine that is required for the survival and activation of memory CD8⁺T cells, natural killer (NK) cells, innate lymphoid cells, macrophages and dendritic cells. IL-15 is implicated in the pathogenesis of various autoimmune diseases such as rheumatoid arthritis, inflammatory bowel disease, psoriasis and autoimmune type 1 diabetes (T1D). IL-15 receptor (IL-15R) consists of a specific α chain, the β chain that is shared with IL-2R and the common γ chain. IL-15 is unique in the manner in which it binds and signals through its receptor subunits. IL-15 that is complexed with IL-15Rα binds to the βγ receptor complex present on the responding cell to mediate its biological effects through a process referred to as trans-presentation. The trans-presented IL-15 is essential to mediate the biological effects on T lymphocytes and NK cells. Here we show that IL-15, but not IL-15Rα, is required for the development of spontaneous and virus-induced T1D, viral clearance and for antigen cross-presentation to CD8⁺ T lymphocytes. Our findings provide insight into the complexities of IL-15 signalling in the initiation and maintenance of CD8⁺ T cell-mediated immune responses.

The common γ-chain cytokine receptor: tricks-and-treats for T cells

Originally identified as the third subunit of the high-affinity IL-2 receptor complex, the common γ-chain (γc) also acts as a non-redundant receptor subunit for a series of other cytokines, collectively known as γc family cytokines. γc plays essential roles in T cell development and differentiation, so that understanding the molecular basis of its signaling and regulation is a critical issue in T cell immunology. Unlike most other cytokine receptors, γc is thought to be constitutively expressed and limited in its function to the assembly of high-affinity cytokine receptors. Surprisingly, recent studies reported a series of findings that unseat γc as a simple housekeeping gene, and unveiled γc as a new regulatory molecule in T cell activation and differentiation. Cytokine-independent binding of γc to other cytokine receptor subunits suggested a pre-association model of γc with proprietary cytokine receptors. Also, identification of a γc splice isoform revealed expression of soluble γc proteins (sγc). sγc directly interacted with surface IL-2Rβ to suppress IL-2 signaling and to promote pro-inflammatory Th17 cell differentiation. As a result, endogenously produced sγc exacerbated autoimmune inflammatory disease, while the removal of endogenous sγc significantly ameliorated disease outcome. These data provide new insights into the role of both membrane and soluble γc in cytokine signaling, and open new venues to interfere and modulate γc signaling during immune activation. These unexpected discoveries further underscore the perspective that γc biology remains largely uncharted territory that invites further exploration.

Anti-CCR4 monoclonal antibody enhances antitumor immunity by modulating tumor-infiltrating Tregs in an ovarian cancer xenograft humanized mouse model

Recent studies have demonstrated that regulatory T cells (Tregs) are recruited to tumor sites where they can suppress antitumor immunity. The chemokine receptor CCR4 is expressed at high levels on functional CD4⁺CD25⁺FoxP3⁺ Tregs and production of the CCR4 ligand CCL22 by tumor cells and tumor-associated macrophages is associated with Treg recruitment to the tumor site. Here, we tested IgG1 and IgG4 isotypes of human anti-CCR4 mAb2-3 for their in vitro activity and in vivo capacity in a NSG mouse model bearing CCL22-secreting ovarian cancer (OvCA) xenograft to modulate Tregs and restore antitumor activity. Both mAb2-3 isotypes blocked in vitro chemoattraction of Tregs to CCL22-secreting OvCA cells. However, they differed in their in vivo mode of action with IgG1 causing Treg depletion and IgG4 blocking migration to the tumors. Primary T cells that were primed with OvCA-pulsed dendritic cells (DCs) demonstrated INFγ secretion that could be enhanced through Treg depletion by mAb2-3. Humanized mice reconstructed with allogeneic tumor-primed T cells (TP-T) were used to evaluate the restoration of OvCA immunity by depletion or blockade of Tregs with mAb2-3. We observed that IgG1 was more potent than IgG4 in inhibiting tumor growth. Mechanism studies demonstrated that mAb2-3 treatment lead to inhibition of IL-2 binding to its receptor. Further studies showed that mAb2-3 induced CD25 shedding (sCD25) from Tregs which lead to a decrease in IL-2-dependent survival. Together, the results demonstrate that mAb2-3 is an agonist antibody that can restore anti-OvCA immunity through modulation of Treg activity.

Trans-presentation of IL-15 modulates STAT5 activation and Bcl-6 expression in TH1 cells

During infection, naïve CD4⁺ T helper cells differentiate into specialized effector subsets based upon environmental signals propagated by the cytokine milieu. Recently, this paradigm has been complicated by the demonstration that alterations in the cytokine environment can result in varying degrees of plasticity between effector T helper cell populations. Therefore, elucidation of the mechanisms by which cytokines regulate T helper cell differentiation decisions is increasingly important. The gamma common cytokine IL-15 is currently undergoing clinical trials for the treatment of malignancies, due to its well-established role in the regulation of natural killer and CD8⁺ T cell immune responses. However, the effect of IL-15 signaling on CD4⁺ T cell activity is incompletely understood. One mechanism by which IL-15 activity is conferred is through trans-presentation via the IL-15 receptor alpha subunit. Here, we demonstrate that differentiated T_H1 cells are responsive to trans-presented IL-15. Importantly, while trans-presentation of IL-15 results in STAT5 activation and maintenance of the T_H1 gene program, IL-15 treatment alone allows for increased Bcl-6 expression and the upregulation of a T_FH-like profile. Collectively, these findings describe a novel role for IL-15 in the modulation of CD4⁺ T cell responses and provide valuable insight for the use of IL-15 in immunotherapeutic approaches.

IL-15Rα is a determinant of muscle fuel utilization, and its loss protects against obesity

IL-15Rα is the widely expressed primary binding partner for IL-15. Because of the wide distribution in nonlymphoid tissues like skeletal muscle, adipose, or liver, IL-15/IL-15Rα take part in physiological and metabolic processes not directly related to immunity. In fast muscle, lack of IL-15Rα promotes an oxidative switch, with increased mitochondrial biogenesis and fatigue resistance. These effects are predicted to reproduce some of the benefits of exercise and, therefore, improve energy homeostasis. However, the direct effects of IL-15Rα on metabolism and obesity are currently unknown. We report that mice lacking IL-15Rα (IL-15Rα(-/-)) are resistant to diet-induced obesity (DIO). High-fat diet-fed IL-15Rα(-/-) mice have less body and liver fat accumulation than controls. The leaner phenotype is associated with increased energy expenditure and enhanced fatty acid oxidation by muscle mitochondria. Despite being protected against DIO, IL-15Rα(-/-) are hyperglycemic and insulin-resistant. These findings identify novel roles for IL-15Rα in metabolism and obesity.

Differential expression of pro-inflammatory cytokines IL-15Ralpha, IL-15, IL-6 and TNFalpha in synovial fluid from rheumatoid arthritis patients

Background

Pro-inflammatory cytokines are directly implicated in the pathogenesis of Rheumatoid arthritis (RA). Variable clinical response to cytokine targeted therapies as TNFalpha and IL-6, strongly highlights the heterogeneity of inflammatory process in RA. Another cytokine, IL-15 has also been related to the inflammatory process in RA. Recently we described for the first time, the presence of its specific receptor, IL-15Ralpha, in synovial fluid (SF). The aim of this work was to compare the expression profile of IL-15Ralpha, its ligand IL-15, TNFalpha and IL-6 and how these cytokines are correlated in SF from RA patients taking as a reference Osteoarthritis (OA), an articular but not autoinmmune disease.

Methods

Synovial fluids were obtained from the knee joints of 60 patients, 30 with confirmed diagnosis of RA and 30 with OA diagnosis. The levels of TNFalpha, IL-6, IL-15 and IL-15Ralpha were measured by ELISA. A statistical analysis was performed with GraphPad Prism v5.0 using the Mann–Whitney U test and Spearman’s rank correlation. A cluster analysis was run in MeV software v4.9.0 and differences across clusters were evaluated by an ANOVA including post-test analysis.

Results

We found higher and significant levels of TNFalpha, IL-6 and IL-15Ralpha but not of IL-15 in RA compared with the OA group. Additionally, a high inter-individual variability in the levels of these 4 cytokines was observed in RA, although we identified 4 patients’ subgroups by cluster analysis of cytokines concentration in SF. We also found a positive correlation between IL-15Ralpha-IL-6 and IL-15Ralpha-IL-15, but not for other pairs of cytokines in RA. In addition we found correlation between the value of IL-15Ralpha in SF and disease activity score, DAS28.

Conclusions

In our current work we found a high inter-individual variability in the levels of TNFalpha, IL-6, IL-15 and IL-15Ralpha in SF of RA patients and were identified four principal clusters of cytokines concentration in SF, suggesting the importance of identifying disease subset of patients for personalized treatment. Finally, we found a correlation between IL-15Ralpha-IL-6, IL-15Ralpha-IL-15, but we did not find any correlation between other pairs of studied cytokines in SF.

Soluble interleukin-15 complexes are generated in vivo by type I interferon dependent and independent pathways

Interleukin (IL)-15 associates with IL-15Rα on the cell surface where it can be cleaved into soluble cytokine/receptor complexes that have the potential to stimulate CD8 T cells and NK cells. Unfortunately, little is known about the in vivo production of soluble IL-15Rα/IL-15 complexes (sIL-15 complexes), particularly regarding the circumstances that induce them and the mechanisms responsible. The main objective of this study was to elucidate the signals leading to the generation of sIL-15 complexes. In this study, we show that sIL-15 complexes are increased in the serum of mice in response to Interferon (IFN)-α. In bone marrow derived dendritic cells (BMDC), IFN-α increased the activity of ADAM17, a metalloproteinase implicated in cleaving IL-15 complexes from the cell surface. Moreover, knocking out ADAM17 in BMDCs prevented the ability of IFN-α to induce sIL-15 complexes demonstrating ADAM17 as a critical protease mediating cleavage of IL-15 complexes in response to type I IFNs. Type I IFN signaling was required for generating sIL-15 complexes as in vivo induction of sIL-15 complexes by Poly I:C stimulation or total body irradiation (TBI) was impaired in IFNAR-/- mice. Interestingly, serum sIL-15 complexes were also induced in mice infected with Vesicular stomatitis virus (VSV) or mice treated with agonistic CD40 antibodies; however, sIL-15 complexes were still induced in IFNAR-/- mice after VSV infection or CD40 stimulation indicating pathways other than type I IFNs induce sIL-15 complexes. Overall, this study has shown that type I IFNs, VSV infection, and CD40 stimulation induce sIL-15 complexes suggesting the generation of sIL-15 complexes is a common event associated with immune activation. These findings reveal an unrealized mechanism for enhanced immune responses occurring during infection, vaccination, inflammation, and autoimmunity.

Highly potent anti-CD20-RLI immunocytokine targeting established human B lymphoma in SCID mouse

Rituximab (RTX), a chimeric IgG1 monoclonal antibody directed against the CD20 antigen, has revolutionized the treatment of B-cell malignancies. Nevertheless, the relapsed/refractory rates are still high. One strategy to increase the clinical effectiveness of RTX is based on antibody-cytokine fusion protein (immunocytokine; ICK) vectorizing together at the tumor site the antibody effector activities and the cytokine co-signal required for the generation of cytotoxic cellular immunity. Such ICKs linking various antibody formats to interleukin (IL)-2 are currently being investigated in clinical trials and have shown promising results in cancer therapies. IL-15, a structurally-related cytokine, is now considered as having a better potential than IL-2 in antitumor immunotherapeutic strategies. We have previously engineered the fusion protein RLI, linking a soluble form of human IL-15Rα-sushi+ domain to human IL-15. Compared with IL-15, RLI displayed better biological activities in vitro and higher antitumor effects in vivo in murine and human cancer models. In this study, we investigated the advantages of fusing RLI to RTX. Anti-CD20-RLI kept its binding capacity to CD20, CD16 and IL-15 receptor and therefore fully retained both antibody effector functions (ADCC and CDC), and the cytokine potential of RLI. In a severe combined immunodeficiency (SCID) mouse model of disseminated residual lymphoma, anti-CD20-RLI was found to induce long-term survival of 90% of mice up to at least 120 days whereas RLI and RTX, alone or in combination, just delayed the disease onset (100% of death at 28, 40 and 51 days respectively). These findings suggest that such ICK could improve the clinical efficacy of RTX, particularly in patients with refractory B-cell lymphoma.

Activated T cells secrete an alternatively spliced form of common γ-chain that inhibits cytokine signaling and exacerbates inflammation

The common γ-chain (γc) plays a central role in signaling by IL-2 and other γc-dependent cytokines. Here we report that activated T cells produce an alternatively spliced form of γc mRNA that results in protein expression and secretion of the γc extracellular domain. The soluble form of γc (sγc) is present in serum and directly binds to IL-2Rβ and IL-7Rα proteins on T cells to inhibit cytokine signaling and promote inflammation. sγc suppressed IL-7 signaling to impair naive T cell survival during homeostasis and exacerbated Th17-cell-mediated inflammation by inhibiting IL-2 signaling upon T cell activation. Reciprocally, the severity of Th17-cell-mediated inflammatory diseases was markedly diminished in mice lacking sγc. Thus, sγc expression is a naturally occurring immunomodulator that regulates γc cytokine signaling and controls T cell activation and differentiation.

IL-15.IL-15Rα complex shedding following trans-presentation is essential for the survival of IL-15 responding NK and T cells

Interleukin (IL)-15 and its specific receptor chain, IL-15Rα, support the development of various effector cells, including NK and CD8 T cells via a mechanism called trans-presentation. Whereas the dynamic of trans-presentation has been shown to involve the recycling of IL-15Rα by presenting cells, the way responding cells integrate, or take advantage of this process has not been evaluated yet. To address this question, we set up a trans-presentation model using a membrane-bound IL-15.IL-15Rα fusion protein, and found that IL-15 is detectable within responding cells following IL-15 trans-presentation. The role of the proteolytic cleavage of IL-15Rα in this process was investigated by generating an uncleavable form of IL-15Rα. We showed that IL-15 entry into responding cells necessitates the cleavage of IL-15.IL-15Rα complex from the surface of IL-15 presenting cells, and observed that IL-15Rα cleavage is associated with a decrease of the duration of Stat5 signaling. Once separated from presenting cells, responding cells are able to recycle IL-15.IL-15Rα complexes via intracellular compartments, for residual proliferation in a time-limited manner. These studies define an unprecedented cytokine pathway in which the IL-15.IL-15Rα complex cleaved from presenting cells allows responding cells to internalize, store and use IL-15.IL-15Rα complex for their own proliferation and survival.

Interleukin-15 and soluble interleukin-15 receptor α in coronary artery disease patients: association with epicardial fat and indices of adipose tissue distribution

Interleukin-15 (IL-15) is a pro-inflammatory cytokine which signals via a specific alpha receptor subunit (IL-15Rα). Increased IL-15 level has been observed in cardiovascular patients and IL-15 immunoreactivity has been detected at vulnerable atherosclerotic plaques. Due to the association between adipose tissue distribution, inflammation and coronary artery disease (CAD), we quantified IL-15 and IL-15Rα in CAD patients with different adiposity and adipose tissue distribution and we evaluated whether epicardial adipose tissue (EAT), a visceral fat depot surrounding and infiltrating myocardium, may be a source of both molecules. IL-15 and IL-15Rα proteins were quantified by enzyme-linked immunosorbent assays. Gene expression of IL-15 and IL-15Rα in EAT depots was evaluated by one colour microarray platform. EAT thickness was measured by echocardiography. Plasmatic IL-15 and IL-15Rα levels were higher in CAD than non-CAD patients. After classification according to adipose tissue distribution, IL-15 was higher in CAD patients with increased abdominal adiposity. Increased level of IL-15Rα was observed both in CAD and non-CAD patients with increased abdominal fat. EAT was a source of IL-15 and IL-15Rα and their expression was higher in CAD patients with increased EAT thickness. In conclusion, our data suggest that circulating levels of IL-15 and IL-15Rα seem to reflect visceral distribution of adipose tissue and that EAT may be a potential source of both IL-15 and IL-15Rα. Future studies on the relationship between IL-15, visceral fat and characteristics of atherosclerotic plaques could help to better understand the complex biology of this cytokine.